1. Field of Invention
The present invention relates in general to nuclear well logging, and pertains in particular to improved methods and apparatus for spectroscopic analysis of inelastic scattering gamma ray energy spectra to provide more accurate information of the composition of earth formations surrounding a well borehole.
2. The Prior Art
Heretofore, various techniques have been utilized to process gamma ray energy spectra for borehole constituent analysis. In the case of inelastic scattering gamma ray energy spectra, it is known that analysis of the spectra to identify the contributions thereto due to carbon and oxygen provides useful information of the presence of oil in a formation.
Additional information concerning the composition of the formation, such as its lithology for instance, is, however, frequently required before an unambiguous determination of the presence of oil can be made. A suitable lithology indicator for this purpose might comprise the ratio of inelastic scattering gamma ray contributions for calcium and silicon.
The derivation of the foregoing information concerning carbon, oxygen, calcium and silicon, and possibly other constituents of the formation and well bore, depends upon accurate constituent analysis of the formation gamma ray spectra. An important and basic technique for performing such analysis is disclosed in U.S. Pat. No. 3,521,064, issued on July 21, 1970, to Moran, et al. In accordance with the Moran et al. teaching, a detected gamma ray energy spectrum for a formation of unknown composition is compared with a composite spectrum made up of weighted standard spectra of the constituents postulated to comprise the formation. The weight coefficients for the standard spectra which give the best fit of the composite spectrum to the unknown spectrum, as determined, for example, by the method of least squares, represent the relative proportions of the constituents in the formation. By appropriate selection and weighting of the standard spectra, the proportions of the constituents of interest, such as carbon, oxygen, calcium, silicon, etc., may be obtained, from which the desired information regarding oil content may be derived.
Although the Moran et al. technique, as disclosed in U.S. Pat. No. 3,521,064, is applicable for the purpose of the present invention and in this respect provides substantial advantages relative to other prior art techniques, the present invention is concerned with methods and apparatus which provide still better results, particularly in connection with the analysis of inelastic scattering gamma ray spectra.
To obtain statistically accurate inelastic scattering gamma ray spectra, it is desirable to irradiate the formation with bursts of neutrons at as high a repetition rate as is practicable so that their time separation is at a minimum. Closely spaced neutron bursts, however, have the disadvantage that gamma rays, originating predominantly from thermal neutron capture reactions between formation constituents and neutrons from one or more preceding bursts, will be present as interfering background during the detection periods for the inelastic scattering gamma rays.
The Moran et al. patent suggests that the capture gamma ray component in the inelastic scattering gamma ray spectrum may be accounted for by predetermining a separate "background" spectrum representive of residual capture gamma radiation from prior bursts. This "Background" would then be included as a standard in the composite spectrum. According to the Moran et al. patent, the standard background spectrum is taken beforehand in a reference borehole or test pit. This, however, does not necessarily reflect the real in situ capture gamma ray spectrum, which varies with change in neutron source strength, sonde environment, sonde performance, etc., and thus may lead to inaccuracies in the constituent proportions obtained from the spectrum matching process.
As evidenced by U.S. Pat. No. 3,780,303 to Smith et al., it has also been proposed in the prior art to detect the level of background gamma radiation in a time interval immediately prior to each neutron burst, and then subtract that level from the inelastic scattering gamma ray counts obtained during the burst.
The background gamma ray count disclosed in the Smith et al. patent, however, reflects only an approximation of the total background level prevailing during the succeeding neutron burst (assuming that the background detection period closely precedes the burst and that the inelastic scattering detection period is short relative to the thermal neutron decay time constant of the formation). Such a count does not afford information of the spectral character or shape of the capture gamma ray spectrum and thus does not accurately compensate the inelatic gamma ray spectrum for the influence of residual capture gamma radiation from prior neutron bursts.
It has further been proposed in U.S. application Ser. No. 869,584, filed Jan. 16, 1978, for R. C. Hertzog et al., now abandoned, and continued as Ser. No. 040,320, filed May 18, 1979, now U.S. Pat. No. 4,317,993, that a background energy spectrum be generated from gamma rays detected during periods between neutron bursts and be utilized to provide one or more standard background spectra for use in the analysis of the inelastic scattering gamma ray spectra in accordance with the aforementioned Moran, et al. practices. The standard background spectra is then updated on a repetitive basis to reflect the current background component in the detected inelastic scattering gamma ray spectrum. The inelastic scattering gamma ray spectrum is thereafter analyzed by comparing it with a composite spectrum, made up of standard spectra of constituents, postulated to comprise the formation, including the background spectra, to determine the proportions in the formation of the postulated constituents.
Finally, it has also been proposed in U.S. Pat. No. 4,232,220 which issued Nov. 4, 1980, to R. C. Hertzog, that the effect of background radiation, within a borehole under investigation, on the inelastic gamma ray measurement, can be accounted for a manner which is substantially independent of the thermal neutron capture time constant of the formation under investigation by taking into consideration variations in the thermal neutron capture gamma ray background spectrum due to changes in environmental parameters in the borehole. A background gamma ray energy spectrum is accumulated during a time interval which immediately follows the time interval in which inelastic intersections are measured. Measurements during this background interval more accurately reflect the thermal background population resulting from lingering thermal neutrons generated during previous high energy neutron pulses and from thermal capture neutrons which are generated within a short time, relative to the neutron capture time constant of the formation, and therefore may be considered as representative of the borehole environment in the vicinity of the detector. Through a simple subtractive step the background information is removed from the measurement obtained during the inelastic gate time interval which is set to encompass the neutron burst, thereby enhancing the inelastic gamma ray measurements derived during the inelastic gate.